PHOENIX — On paper, Arizona's controversial new immigration law is not that different from the federal version. But the key difference is this: Arizona wants every illegal immigrant caught and deported. The federal government says treating all 11 million of the nation's illegal immigrants as criminals would overwhelm the system. In its lawsuit challenging the Arizona law, the Justice Department says its policy is to focus on dangerous immigrants: gang members, drug traffickers, threats to national security. Law-abiding immigrants without documentation would largely be left alone. Homeland Security officials say the government cannot possibly find, arrest and deport everyone who is here illegally. And trying to do so would also upset a balance crafted by Congress that takes into account humanitarian interests and foreign relations. But proponents of the Arizona solution insist that's no reason not to try. And they say the state's toughest-in-the-nation law is a reasonable way to start. "If it's really the case that they don't have enough resources to enforce the laws that Congress has passed, it would seem it's incumbent on them to go back to Congress and ask for more resources," said Steven Camarota, research director at the center for Immigration Studies, a group that favors stricter enforcement of immigration laws. "But since they don't do that, it sort of undermines the argument." Arizona's new law is nearly identical to federal immigration law. At issue is how it is enforced. The federal government says the state law is unconstitutional because it usurps federal authority to protect U.S. borders and American citizens. Arizona counters that the federal government is not doing its job, which forces state officials to step in. State lawmakers argue that the federal government already enlists local authorities to identify illegal immigrants who have been arrested for other crimes. The new law, they say, just extends that to police patrols. The federal government says the law goes too far by making it a state crime to be in Arizona illegally and requiring police to question the immigration status of anyone they encounter who is believed to be undocumented. The furor over the Arizona law is overblown, Camarota said Wednesday. It does not envision mass deportations or roundups, just a slow but steady pressure on illegal immigrants to leave Arizona — either for their home countries or for another state. The number of illegal immigrants in the country fell for the first time this decade in 2007, and dropped another 800,000 between 2008 and 2009, primarily due to the recession and increased enforcement efforts. As of January 2009, an estimated 10.8 million people were in the country illegally, 1 million less than the 2007 peak, according to the Department of Homeland Security. Deportations have been increasing, climbing from 185,944 in 2007 to 387,790 last year. Many critics argue the federal government cannot selectively enforce immigration law, but it's common for law enforcement at all levels to prioritize. Small-time pot dealers do not receive the same level of investigation or prosecution as big-time heroin traffickers. The government has also tolerated medical marijuana in 14 states. But Arizona's law has brought selective enforcement — and the differences that exist even among police agencies — into clearer focus. Those differences are stark, even in the Phoenix metro area. Phoenix Police Chief Jack Harris says in an affidavit supporting the federal suit that he will probably have to move detectives focused on violent crime to street patrol because regular officers will be busy enforcing Arizona's new law. But Maricopa County Sheriff Joe Arpaio, who has been at the forefront of the effort to empower local authorities to enforce immigration laws, routinely assigns deputies to crime sweeps where they target illegal immigrants. The federal government is worried that other states will follow Arizona's lead, overwhelming federal agencies with non-criminal illegal immigrants who will cost the government millions to deport. A March study by the liberal Center for American Progress estimated that deporting the entire illegal immigration population and securing the borders would cost $285 billion over five years. In the government lawsuit, officials with Immigration and Customs Enforcement and Customs and Border Protection declared they will be forced to shift resources from major cases to minor ones if the law goes into effect as scheduled on July 29. Five other lawsuits, filed by immigrant-rights groups, the American Civil Liberties Union and individuals, are already before a federal judge in Phoenix. The federal challenge filed Tuesday is expected to be transferred to the same judge, who has hearings set for next week on requests to block the law from taking effect. The federal lawsuit focuses on a core constitutional concern — balancing power between the states and the federal government. More specifically, the issue centers on the long-running "pre-emption" legal argument that says federal law trumps state law. The government sidestepped concerns about the potential for racial profiling and civil rights violations most often raised by immigration advocates. Experts said those are weaker arguments that do not belong in a federal legal challenge.